Activated carbon, alumina, zeolites, and the like, are widely used in filtration appliances. These types of materials are sometimes referred to collectively as “active particulate”—see U.S. Pat. No. 5,696,199 to Senkus et al.—because of their configuration and innate ability to interact with fluids by sorbing (adsorbing and absorbing) components in the fluid. Their good filtration properties arise from a highly porous or convoluted surface structure, which provides an increased surface area.
Activated carbon, in particular, is widely used to protect persons from inhaling a variety of toxic or noxious vapors, including poisonous gases, industrial chemicals, solvents, and odorous compounds. Its surface porosity typically results from a controlled oxidation during manufacture. Activated carbon is derived, for example, from coal or coconut shells and can be produced in the form of powders, granules, and shaped products. It is commonly used in individual canisters or pads for gas masks. Important properties of commercial activated carbon products include those related to their particle size as well as their pore structure.
Although commonly used in many filters, activated carbon does not have a great capacity to remove gases and vapors that have low boiling points. Treatments therefore have been devised where chemicals are placed on the carbon surface and within the carbon pores to provide enhanced filtration capabilities for such gases and vapors. These treatment processes are generally known as “impregnation” methods, and the result of the treatment is an impregnated activated carbon.
Various impregnants have been developed for removing a wide range of chemicals. In industry—where the nature of the hazard is known in advance—the practice has been to select an appropriate filter for the known hazard. Consequently, filters that are effective at removing a specific chemical type or class are often used in industrial applications.
Over time, regulatory structures for the selection and use of respiratory protective equipment have evolved, along with approval systems, to ensure that the equipment designs are capable of meeting necessary performance requirements. The European Standard (EN) system has been adopted widely in Europe and elsewhere. Another example is the National Institute for Occupational Safety and Health (NIOSH), which has been adopted in the USA, Canada, and certain other countries.
For emergency responders, filtration-based protection systems are appropriate for personnel undertaking various tasks close to a point of a chemical release. Although a quick response is often desirable, delay may be inevitable if the responders need to first identify the toxic component in the surrounding air in order to select an appropriate filter. To avoid carrying an inventory of many different filters, it is beneficial to have one filter type, which can provide protection against various hazards.
The first U.S. patent that described a treated carbon, which removed a variety of gases arose from developments to protect personnel in World War I battles where chemical agents were used. The 1924 patent by Robert E. Wilson and Joshua C. Whetzel (U.S. Pat. No. 1,519,470) describes several methods of impregnating granular activated carbon with metals and their oxides. For most purposes, the preferred impregnants were metallic copper and copper oxides. The products made by the method described in this patent later became known as “whetlerites”. Workers at Edgewood Arsenal made subsequent progress in techniques for copper impregnation, and by the early 1940s, a copper impregnated carbon, designated “Type A” whetlerite, was the standard canister fill for US military masks. This sorbent was prepared by treating activated carbon with copper ammine carbonates in an ammonia solution. When the treated carbon was heated to 150° C. or higher, the ammine carbonates decomposed to form copper oxides in the carbon pores. Ammonia and carbon dioxide were liberated during drying. The oxide impregnant acted as an oxidizing and basic medium for the retention of acidic or oxidizable gases and vapors. The preparation of an impregnated carbon particulate of this type is described in U.S. Pat. No. 1,956,585 to Oglesby et al. It is now known, however, that active particulates treated with such copper compounds can react with hydrogen cyanide (HCN) to generate cyanogen (NCCN), an equally toxic gas. Other variations on the Wilson et al. technique have been developed—see, for example, the following patents: U.S. Pat. No. 2,920,051, DE 1,087,579, FR 1,605,363, JP 7384,984, and CZ 149,995.
In a particular advance over the 1924 Wilson et al. activated carbon, chromium (VI) salts were used to aid in removal of the NCCN generated by reaction of HCN with copper based salts—see U.S. Pat. Nos. 1,956,585 and 2,920,050. In recent years, however, the use of such Cr-based materials has been limited by both environmental and health concerns. An in depth report on impregnant formulations can be found in “Military Problems with Aerosols and Nonpersistent Gases”, Chapter 4: “Impregnation of Charcoal”, by Grabenstetter, R. J., and Blacet, F. E., Division 10 Report of US National Defense Research Committee (1946) pp. 40-87. The favorable properties obtained by using chromium also can be realized by the use of metals such as molybdenum, vanadium, or tungsten. Whetlerites containing these metals are described in several patents, including U.S. Pat. Nos. 4,801,311 and 7,004,990.
Subsequent research also has explored shelf life improvements using impregnated organic compounds on carbon. One material found to improve shelf life towards a cyanogen chloride (CK) challenge is triethylenediamine (also known by other names such as TEDA, DABCO, or 1,4-diazabicyclo[2.2.2]octane). When impregnated on carbon, TEDA has been found to be capable of reacting directly with cyanogen chloride and to be capable of removing methyl bromide and methyl iodide.
In 1993 Doughty et al. (U.S. Pat. No. 5,492,882) described the use of copper carbonates and sulfates in the presence of zinc and molybdenum oxides. This formulation was an advancement over the Wilson et al. work and others because it incorporated molybdenum oxides, which increased capacity for the HCN reaction product NCCN and avoided the use of chromium found in earlier versions. In addition, Doughty et al. removed basic gases such as ammonia. As discussed below, the Doughty et al. method is generally limited by its utilization of ammoniacal solutions and salts during the impregnation process. These chemicals and solutions are generally expensive to handle on a manufacturing scale because specialized ventilation and scrubbing equipment is required to meet health and safety concerns and environmental release standards.
In U.S. Pat. No. 7,309,513, Brey et al. presented an advance by replacing the relatively expensive molybdenum compounds with tungsten while retaining the ability to remove NCCN and other basic gases such as ammonia. Despite this advance, Brey et al. also relied on the use of ammonia-based salts and solutions to impregnate metal into activated carbon and other substrates.